Axions could be generated mainly by Bremsstrahlung processes and
Compton scattering [1]}, originally proposed for star
cooling via axion emission, see e.g. [2]} and a
more recent work [3]} for Bremsstrahlung processes
and Compton scattering studied a long time ago
[4]} and more recently [5]}.
Also generated via the Primakoff conversion of photons to axions
in the presence of a magnetic field [6]}. All these
yield a continuous axion spectrum, the Primakoff conversion with
an average energy around 4 keV [7]}. They can also be
generated by nuclear transitions, which involve monochromatic
axions, e.g. 14.4 keV in the case of the
de-excitation of \(^{57}\) Fe* and 5.5 MeV in the case \(p+d\rightarrow ^3_2\) He*.
Axions produced in nuclear processes are mono-energetic because their
energies correspond to the transition energies to specific
nuclear states. These solar axions can be emitted and escape from
the solar core due to the very weak interaction between the axion
and matter.
